Two rocks passing each other

A rock is thrown upward from ground level with an initial velocity of 15 m/s while at the same instant another rock is thrown downward from a bridge that is 20m above the ground. The second rock has an initial velocity of 8 m/s downward. Ignore air resistance. Find the time when they pass by each other. How high above the ground are the rocks when they pass each other? How fast are the rocks moving when they pass each other?

For time, I ended up getting .204s but then again I was making assumptions when I did that calculation. Can anybody point me in the right direction?

A rock is thrown upward from ground level with an initial velocity of 15 m/s while at the same instant another rock is thrown downward from a bridge that is 20m above the ground. The second rock has an initial velocity of 8 m/s downward. Ignore air resistance. Find the time when they pass by each other. How high above the ground are the rocks when they pass each other? How fast are the rocks moving when they pass each other?

For time, I ended up getting .204s but then again I was making assumptions when I did that calculation. Can anybody point me in the right direction?